Current methods for measuring dietary intake in free-living people typically result in an average under-estimation of energy intake (EI) of 10-50%. Data are suggestive, but not conclusive, that EI underreporting is specific to foods and beverages high in energy density (i.e., high in sugar and/or fat). Our work and that of others has shown that dietary misreporting, that is, underreporting or overreporting, may result in spurious statistical associations of dietary variables with body mass index (BMI) and chronic disease risk factors, which in turn may lead to incorrect conclusions about potential dietary contributors to overeating, obesity, and other health outcomes. Biomarkers that serve as objective, independent measures of dietary variables thought to be commonly and selectively misreported would help to verify which specific dietary factors are misreported by which individuals. In this response to PAR-07-259, Improving Diet and Physical Activity Assessment, our overall hypothesis is that dietary misreporting is specific to certain types of foods and/or nutrients;in particular, foods high in sugar and/or fat are underreported, and fruits and vegetables are overreported. We will recruit n=181 ethnically diverse men and women aged 18-65 y and BMI 18.5-45 kg/m2 primarily from the Healthy Purdue population--a university-based healthy lifestyle program with >9000 participants--but also from other sources. We will test our hypotheses by assessing a combination of novel (urinary sucrose + fructose for foods high in sugar) and established (plasma carotenoids and serum phylloquinone for fruits and vegetables) dietary biomarkers, reported dietary intakes, and their relationships with each other as well as with health outcomes in the total study sample (n=181) compared to the subsample of physiologically plausible EI reporters (target ne120;where, according to previously-established methodology, an individual's EI will be regarded as plausible if it is within [unreadable]1 SD of total energy expenditure measured by doubly labeled water). We expect the results of this study improve our understanding of the nature of reporting error in dietary assessment methodology, and ultimately help clarify relationships between different components of dietary intake and health outcomes.